The invention relates to a radio key for an access system of a vehicle. In particular, the invention relates to a radio key (also referred to as ID transponders) for so-called passive access systems such as keyless entry systems.
Various vehicle keys for passive access systems are known from the prior ad. By way of example, a vehicle key from the family of the inventive vehicle keys is described in U.S. Pat. No. 7,602,274 B2. The vehicle key has a coil assembly configured as a so-called 3D coil assembly for signal reception in the low frequency (LF) frequency range. Three antenna coils are oriented in different spatial directions, in order to be able to receive a low frequency signal transmission (e.g. at 125 kHz) from the vehicle when the vehicle key is in any position.
Such a 3D coil assembly is coupled to an associated receiver circuit. The key also has a control circuit, which in turn is coupled to the receiver circuit. The low frequency signal transmission from the vehicle to the vehicle key is used to wakeup the vehicle key, in particular its control circuit, from a resting state, and initiate a subsequent communication in the high frequency range. For this, the vehicle key also has at least one high frequency antenna configured for high frequency communication in addition to the LF antennas, and an associated transmission circuit.
Because the energy resources for the mobile radio key should be conserved, the vehicle key should not be activated too frequently. For this reason, the central control circuit of the radio key, which contains a microcontroller and a dedicated means for executing program code is only woken up when the receiver circuit receives an appropriate signal in the low frequency range via the 3D coil assembly.
Products are available an the market that combine numerous of the aforementioned components to form modules, and more or less integrate said. By way of example, products from Texas Instruments are known, which integrate coil assemblies with receiver circuits (TI car access product family).
The keys of the specified type are provided in particular for so-called polling, i.e. systems querying the surroundings at regular intervals from the vehicle via LF signals.
With such systems that have numerous LF receiver coils, it is normal to select one of the coils for a subsequent signal reception for decoding the low frequency signals when a low frequency signal has been received by the 3D coil assembly. This selection can be in accordance with various criteria. It has proven, however, to be the case that such an approach for selecting a coil is associated with problems regarding the performance and reliability of the coil selection.
The object of the invention is to improve the reliability of the performance of a radio key with a 3D coil assembly.